The idea that an electrode surface could be "molecular-engineered" via chemical modification such that desirable features of the selected modifier can be translated to the target surface has both fundamental and practical significance. Polymer matrices have proven to be particularly useful for immobilizing interesting chemicals on electrode surfaces. One particular type of chemical modification, which is significant in terms of energy conversion and waste disposal application possibilities, involves the use of dispersed metal clusters of aggregates as catalysts on the electrode surface. Such heterogeneous catalyst systems, however, suffer from two problems: (a) First, the loss of dimensionality of the catalyst particles as a result of their surface immobilization has a deleterious effect on reaction cross-sections. Thus, the effective (maximal) catalyst loading is limited to a monolayer at best. (b) Second, the methods that have been commonly employed for the deposition of these metal catalysts on the electrode surface (e.g. electrodeposition) result in rather poor control of the size distribution (i.e. dispersion) of the catalyst particles. It is well known that catalytic applications require these particles to have colloidal dimensions in order to be most effective. Of course, homogeneous catalyst systems do not suffer from the aforementioned two problems. However, a key practical impediment exists in the use of homogeneous catalysts, viz. that of catalyst recovery and regeneration in the reactor after use. In contrast, the use of heterogeneous catalysts does not present this difficulty.
The vast majority of the prior work relies on the deposition of catalyst particles onto a preformed polymer film. This approach differs in two important respects from that of the present invention. First, the catalyst particles are incorporated in the present inventors' scheme into a growing polymer framework. This insures a 3-D array of catalyst particles. Second, the catalyst is not electroreduced from a precursor as in the majority of the work described below. Instead, the present inventors believe that the (negatively charged) Pt.sup.0 colloidal particles migrate to the (positively charged) anode and are then trapped within the forming polymer matrix. The present inventors term this an "electrotrapping" mechanism. In this respect, Kawai et al., Yoneyama et al. and Beck et al. describe some synthetic semblance. However, both the nature of the incorporated particles (oxides in these references and metals or metal alloy catalysts in the present invention) and the end goals of the studies differ. Liu et al. also utilize a colloidal catalyst suspension (like the present inventors do) but their polymers are chemically very different and thus the ultimate synthetic procedure is variant in the two instances.
Noufi et al. studied the evolution of O.sub.2 at polypyrrole electrodes containing colloidal RuO.sub.2 particles. The latter were incorporated into the polymer during synthesis as RuO.sub.4.sup.2- "dopant" anions and subsequently reduced to generate RuO.sub.2 in situ. This synthesis method obviously differs in that the present inventors' Pt colloid particles (as a model example) are trapped within a growing polymer matrix not as dopant ions. No subsequent catalyst generation step is needed in the present inventor's approach. The particle size distribution of RuO.sub.2 (a very important parameter in catalytic activity) further is not known in the above-cited study.
Liu et al. describe the incorporation of "finely divided and dispersed" Pt particles in polymers such as Nafion.RTM.. It is also claimed that the scheme may be extended to other matrices such as metals, metal oxides, clays, minerals and "other types of conducting and non-conducting solids". The preparation consists of mixing solutions of the polymer (e.g. Nafion.RTM.) and a colloidal suspension prepared according to a previus procedure, Hirai et al. Both the preparation procedure as well as the polymer differ from the present inventors' methodology although the end application of the two types of materials could well overlap.
Tourilion et al. (a) describe the electrochemical inclusion of metallic clusters in organic conducting polymers with poly 3-methylthiophene (a conductive polymer) and Cu as specific examples. The focus of the study is the characterization of these clusters using in situ dispersive X-ray absorption. The major difference lies with the catalyst incorporation procedure in that the metal ions are electroreduced onto a pre-existing polymer framework. This is what the present inventors refer to as the "2-D architecture" in the present application.
Tourilion et al. (b) describe a very similar study except that catalysts such as RuO.sub.2, Ag, and Pt also are considered. The focus of the present study is on the hydrogen evolution reaction (here and elsewhere to be abbreviated as HER).
In Kao et al., the electrochemical dispersion of Pt microparticles in polymeric matrices such as poly(vinylacetic acid) is described. The points of departure from the present inventors' approach include the type of polymer (non-electronically conductive), method of catalyst incorporation (not electrotrapped but electroreduced, see above), and the polymer thickness (very thin, 400-1000 .ANG. compared to that of the present invention).
Weisshaar et al. describe a similar study but it also includes Pd, Ag, Ni, Cd and Pt-Pd catalysts.
Daube et al. is a comprehensive review of the work of Wrighton et al. on the incorporation of catalyst (mainly Pd) particles in viologen-based redox polymers. The polymers considered are chemically very different from ours. The method of catalyst attachment (which occurs by electrostatic binding onto the polymer backbone) in the Wrighton work also is fundamentally different from the present inventors' approach.
In Bartak et al. (1986), platinum "microparticles" were electrodeposited at three types of poly(4-vinyl pyridine) films on glassy carbon electrodes. Again the catalyst was incorporated into a pre-existing polymer network by soaking the latter with PtCl.sub.6.sup.2- followed by electroreduction of the latter.
In Vork et al. (1986), two methods are described for the dispersion of Pt particles in polypyrrole: (a) electrodeposition of Pt onto a preformed polymer (see above) and (b) the incorporation of Pt particles during the polymerization of pyrrole. Method (a) has been discussed earlier. In Method (b) 44 .mu.m Pt particles were used as "fine powdered metal dust" and not as colloidal catalyst as in the present inventors' methodology. These particles obviously are too large to be catalytically useful (see Mukerjee).
In Chandler et al., metals (e.g., Pd, Pt and Pb) are electrodeposited onto a preformed polypyrrole film.
Vork et al. (1987) is a study similar to York et al. (1986), but the focus of the study here was to monitor the ohmic resistance of the Pt-loaded polymer.
Itaya et al. have electroreduced Pt catalyst particles onto a Nafion.RTM. polymer network and have measure the HER. Both the type of polymer and the method of catalyst incorporation differ from the present inventors' approach.
Three types of Pt-loaded polypyrrole are considered in Holdcroft et al. Their "Type II" electrodes are similar to those discussed above. Type III and IV electrodes were prepared by immersing pre-reduced and oxidized polymer films in PtCl.sub.6.sup.2- solution. Again, these methods are distinctly different from the one the present inventors employ. These authors do claim, however, to have achieved a 3-D catalyst distribution in Types III and IV electrodes (as verified by Auger electron spectroscopy depth profiles). No catalyst particle size information is presented. The conclusions of these authors (namely that of poor substrate permeation, c.f. p. 102) are also at variance with the present inventors' conclusions. The present inventors observe no substrate mass transport limitations in their studies and systems.
Bartak et al. (1988) have utilized methodology as in Kao et al., Weisshaar et al., and Bartak et al. (1986), but for a conductive polymer (polyaniline) instead. Reactions studied included HER and methanol oxidation. The catalyst incorporation strategy (as in the earlier studies by this group) differs in a fundamental sense from the present inventors'.
In Vork et al., again, a preformed polypyrrole layer is used (unlike in the present inventors' approach), and Pt catalyst deposited by constant current electrolysis of a H.sub.2 PtCl.sub.6 solution. The authors use the resultant polypyrrole/Pt system for the study of oxygen reduction. Such electroreductive platinum deposition yields Pt particles greater than 100 nm in size.
Shimazu et al. use methodology and systems very similar to that described earlier in Itaya et al.
In Gholamian et al., platinum is loaded onto a preformed polyaniline by potentiodynamic cycling in H.sub.2 PtCl.sub.6. The method of preparation and specific systems considered obviously are different from those of the present invention.
Kawai et al. report the electrochemical synthesis of polypyrrole with incorporated TiO.sub.2 particles. Pyrrole was electrolyzed in the presence of illuminated TiO.sub.2 and also in the presence of aqueous TiO.sub.2 suspensions. The end goal (unlike the approach of the present inventors) of this study is to impart photosensitivity to the polypyrrole.
Yoneyama et al. (1990) utilize WO.sub.3 particles instead for incorporation into polypyrrole. Similar comments apply as in Kawai et al.
Tian et al. describe electrochemical and XPS studies of silver clusters in polyaniline films. The method of catalyst incorporation rather similar to Noufi in that Ag ions are introduced as metal complex (thiocyanide) ions and then subsequently electroreduced to generate Ag metallic clusters.
Wessling et al. propose to use convention filler materials (which are used in admixture with conventional polymers, e.g., rubber) along with polypyrrole. There is little parallel to the invention here either in the preparation details or with the end applications.
In Leone et al., palladium particles were incorporated by electroreducing PdCl.sub.2 onto preformed polypyrrole and polyaniline films.
In Hable et al., Pt/Sn bimetallic catalysts were electrodeposited onto a preformed polyaniline layer from aqueous solutions containing Pt(IV) and Sn(IV).
Beck et al. describe an extension of the approach in Kawai et al. and describes the dispersion of TiO.sub.2 particles in polypyrrole. Photoelectrochemical properties of the resultant composite are explored. Similar comments as to Kawai et al. apply to the relation of this study to the proposed invention.
Hinden et al. relate to a catalytically modified corrosion-resistant valve-metal (e.g., Ti, Zr, Ta, Nb) which is then embedded in a support electrode (e.g., Pb-Ag alloy) matrix. The catalysts include the platinum group metals or the corresponding oxides. An "electrically conductive polymer" (e.g., polyacrylonitrile) is used as a precursor. The electrode structure, the method of preparation outlined in this invention, and the component materials thereof all are fundamentally and chemically different from the present inventors' approach.
In Kuwana et al., the concept and the approaches described are related to the material discussed above in relation to Kao et al., Weisshaar et al., Bartak et al., (1986) and Bartak et al. (1988) from the same research group.
Polymer films on electrode surfaces containing dispersed metallic clusters or aggregates (Bookbinder et al.; Dominey et al.; Bruce et al.; Simon et al. (1983 and 1985); Harrison et al.; Stalder et al.; Pickup et al.; Liu et al.; Tourilion et al. (b); Tourilion et al. (a); Yasser et al.; Weisshaar et al.; Bartak (1988); Lyons et al. (a); Lyons et al. (b); are of both practical and fundamental importance. Aside from their useful catalytic activity towards technologically important substrates, these microheterogeneous assemblages also provide unique opportunities for exploring novel types of catalyst-support interactions. For example, polyaniline and Nafion containing dispersed Pt microparticles have been reported to show significant enhancement in catalytic activity, relative to bulk Pt, towards the oxidation of methanol (Bartak et al. (1988), Gholamian et al., Shimazu et al.). Electronically conductive polymers such as polyaniline and polypyrrole are particularly attractive host media for the confinement of catalyst particles (Tourilion et al. (b), Burtak et al. (1988), Gholamian et al., Chandler et al., Vork et al. (1986), Vork et al. (1990), Holdcroft et al., Bedioni et al.). These media potentially provide an efficient route for the shuttling of electronic charge to the catalyst centers, especially in reactions spanning a potential regimen wherein the polymers retain their electronic conductivity. However, prior studies on conductive polymer/catalyst composites have largely utilized catalyst particles in the micrometer size range (Gholamian et al., York et al., Holdcroft et al.). It is well known that particle size plays a crucial role in catalytic activity (Mukerjee). The present inventors describe a novel route to the electrosynthesis of polypyrrole/thin films containing dispersed Pt particles in the catalytically useful nanometer size range. Unlike the electroreduction method employed in most previous studies (i.e., PtCl.sub.4.sup.-2 or PtCl.sub.6.sup.-4.fwdarw.Pt.sup.0), the present inventors have used a colloidal dispersion of Pt.sup.0 (Mills et al.; Furlong et al.) in conjunction with the pyrrole-based polymerization medium to generate polypyrrole/Pt thin films. The Pt colloidal particles apparently are "electrotrapped" within the growing polymer matrix affording a three-dimensional array of catalyst within it (c.f. FIG. 1).
This electrosynthesis process was monitored at an underlying Au electrode using quartz-crystal microgravimetry. The use of this latter technique in the electrochemical arena has rapidly grown in recent years; reviews are available (Thompson et al; Schumacher). Using this technique, the present inventors were able both to monitor the dynamics of thin film growth as well as to quantify the uptake of Pt catalyst by the polymer. The present inventors are not aware of similar studies on chemically modified electrode surfaces using this powerful analytical tool.
Finally, the present inventors demonstrate via hydrodynamic voltametry the unusual catalytic activity of their polypyrrole/Pt (ppy/Pt) films towards the reduction of dioxygen and protons. In particular, the present inventors highlight the enhancement in catalytic activity obtained by distributing the Pt colloids in a three-dimensional array with in the polypyrrole matrix (FIG. 1). Thus, unlike with the usual two-dimensional (surface) catalysis situations (FIG. 1), thicker electrodes show higher catalytic activity towards the above model substrates as opposed to thinner samples. The three-dimensional distribution of Pt.sup.0 particles within the polypyrrole matrix also facilitates the efficient shuttling of charge from the (underlying) support electrode to the reaction site. This feature becomes especially important when a reaction of interest occurs in circumstances where the polymer is a poor electronic conductor.